Many investigators and researchers use mice, hamsters, and other rodents or creatures in their experimentation and research. Often, the creatures must be housed before, during, or after the course of the research or experimentation, for example, for preparation or observation. Cages fabricated with plastic or other polymeric material have been used for this purpose. Such cages typically are equipped with internal wire bar lids and top covers or "microbarrier tops." The wire bar lids are useful in retaining food, and the microbarrier tops cover the cage and typically have a plurality of permeable sections and/or filters in their top surfaces for filtering the air or providing air inside the cages.
Additionally, it is important that the cages be stackable or capable of being nestled inside of one another for compact storage. The cages typically are configured to fit or slide within rack units, and these rack units may be connected to sources for automatically providing water, food, or air to the creatures inside the cages.
To illustrate, FIG. 1 shows a perspective view of a prior art cage apparatus 10 having a bottom receptacle 20, a wire bar lid 30, and a microbarrier top 40. FIG. 2A shows a cross-sectional length-wise view of the receptacle 20 and microbarrier top 40 taken along the line A--A of FIG. 1; FIG. 2B shows a cross-sectional width-wise view of the receptacle 20 and microbarrier top 40 taken along the line B--B of FIG. 1; and FIG. 2C shows a cross-sectional side view of the wire bar lid 30 taken along the line A--A of FIG. 1. As can be seen from these Figures and as will be explained in further detail later, the receptacle 20 has an upper rim 21; the wire bar lid 30 has a flat edge 38 that encircles its periphery; and the microbarrier top comprises a centrally-disposed promontory 42 and a surrounding lip portion 46. These parts (receptacle, lid, and top), are sized and configured so that the rim of the receptacle 21 will hold the wire bar lid 30 and microbarrier top 40 in place on the receptacle; the edge 38 of the lid 30 rests against the rim 21, and the lip 46 of the microbarrier top 40 snap-fits over the rim 21. As a further illustration, reference is made to the following U.S. patents which show animal cages having an open top body (receptacle), a wire bar lid, and a filter cap (microbarrier top). See U.S. Pat. No. 4,640,228 issued Feb. 3, 1987 to Sedlacek, entitled "Animal Cage Assembly With Reusable Filter Cap"; U.S. Pat. No. 4,480,587, issued Nov. 6, 1984, also to Sedlacek and entitled "Animal Cage Assembly With Reusable Filter Cap"; and U.S. Pat. No. 5,165,362 issued Nov. 24, 1992 to Sheaffer et al. entitled "Ventilated Cage and Open Rack System."
Such cage apparatuses have been sold for many years, and users naturally have accumulated inventories of racks and cages, including the wire bar lids and microbarrier tops to be used with the receptacles. These items are fabricated to be durable and permit for re-use and refurbishing. See, e.g., U.S. Pat. Nos. 4,480,587 and 4,640,228, referenced recommended square-inch space allocations for housing commonly-used laboratory rodents. The NRC recommended an increased square-inch space allocation, that is, many prior art cages having the receptacles, wire bar lids, and microbarrier tops as previously described have a smaller square-inch space allocation than was recommended in the Guide.
Since the publication of the Guide, the laboratory cage industry has attempted to provide new cage designs with larger square-inch space allocations. In 1996, Lab Products Inc. advertised a mouse cage which it claimed in advertisements houses five mice while complying with the space requirements of the Guide. In increasing the space allocation of this cage, however, the dimensions of the cage receptacle were changed (i.e., increased), such that new wire bar lids and microbarrier tops would have to be purchased to fit the newly-sized cages.
Thus, there is a need for a new cage design enhancing the floor space of the cage receptacle as compared with prior art cages, while maintaining compatibility with existing wire bar lids and microbarrier tops. Further, there is a need for a cage that achieves this greater floor space but is also stackable. Preferably, the cage should be comprised of a single, integral unit, avoiding the need for the assembling or disassembling of parts prior to storing or stacking.